s-Process in low metallicity Pb stars

Mem. S.A.It. Vol. 77, 985

SAIt 2006 c Memorie

della

s-Process in low metallicity Pb stars

S. Bisterzo ^1,2 , R. Gallino ^2,3 , O. Straniero ⁴ , I. I. Ivans ^5,6 , F. K¨appeler ¹ , and W. Aoki ⁷

1

Forschungszentrum Karlsruhe, Institut f¨ur Kernphysik, D-76021 Karlsruhe, Germany

2

Dipartimento di Fisica Generale, Universit´a di Torino, via P. Giuria 1, 10025 Torino, Italy

3

Center for Stellar and Planetary Astrophysics, School of Mathematical Sciences, PO Box 28M, Monash University, 3800 Victoria, Australia

4

Osservatorio Astronomico di Collurania, INAF, Teramo, 64100, Italy

5

The Observatories of the Carnegie Institution of Washington, Pasadena, CA, USA

6

Princeton University Observatory, Princeton, NJ, USA

7

National Astronomical Observatory, Tokyo, Japan

e-mail: bisterzo@ph.unito.it, sara.bisterzo@ik.fzk.de

Abstract. We consider a sample of very metal-poor, C-rich, s-rich and lead-rich stars observed at high-resolution spectroscopy, and some recent spectroscopic data of C+s-rich stars obtained at moderate resolution. The spectroscopic data of these stars are interpreted with AGB theoretical models of different

¹³

C-pocket efficiencies, initial mass and initial r-enrichment. When lead is not measured we give our theoretical prediction. The observed stars are not on the AGB phase, but are main sequence or giant stars. They acquired the C and s enrichments by mass transfer in a close binary system from the more massive companion while on the AGB (now a white dwarf). A considerable fraction of the stars show both high s and r enrichments. To explain the s+r enriched stars we assume a parental cloud already enriched in r-elements. The measurement of Nb is an indicator of an extrinsic AGB in a binary system. The intrinsic indicator [hs/ls] constrains the initial mass, while [Pb/hs]

and [Pb/ls] are a measure of the s-process efficiency. The apparent discrepancies of C and N abundances may be reconciled by assuming a strong cool bottom process occurring during the AGB. An important primary production of light elements, from Ne to Si, increasing with the star mass, is predicted for AGB models at very low metallicity, induced by n capture on primary

²²

Ne and its progenies.

Key words. Stars: C and s rich – Stars: abundances – Stars: Population II – Stars: nucle- osynthesis

1. Introduction

We analysed a sample of very metal-poor stars ([Fe/H] < −2) showing an enhancement in C, s elements, and lead, in several cases also in r- elements. We give an interpretation of spectro- scopic abundances using AGB models of dif-

Send offprint requests to: S. Bisterzo

ferent s-process efficiencies, initial mass and

initial r-enrichment (see Gallino et al. 2005 for

further details). The estimated initial masses

are in the range 1.2 to 1.5 M ; the

¹³

C-pocket

is assumed to be constant pulse by pulse. We

consider a large range of

¹³

C-pocket efficien-

cies, starting from the ST case of Gallino et

al. (1998) and multiplying or dividing the

¹³

C

abundance in the pocket by different factors.

For giant stars the fit was obtained by further introducing a proper dilution factor to simu- late the mixing effect of the AGB winds with the convective envelope of the observed star.

To reproduce stars with both s and r enhance- ments, since the AGBs do not synthesise the r elements, we assumed that the parental cloud of the binary system was already enriched in r elements. The choice of the initial r-rich iso- tope abundance normalised to Eu is made con- sidering the r-process solar prediction from Arlandini et al. (1999). Moreover, we selected the most significant C and s-rich stars from the sample of Barklem et al. (2005). In these cases the lead is not measured, and we give our (very high) predictions.

2. Comparison between theory and observations

All the stars considered in this paper are re- ported in Tables 1 and 2. In Tab. 1, fourth col- umn, the intrinsic indicator [hs/ls] may con- strain the initial mass, while in the fifth col- umn, [Pb/hs] is an indicator of the s-process efficiency. In the ninth column the value [Eu/Fe]

ⁱⁿⁱ

corresponds to the assumed initial r-process enrichment in the progenitor cloud.

The observed [Eu/Fe] is indicated by a star symbol. These values correspond to the as- sumed initial r-process enrichment [Eu/Fe]

ⁱⁿⁱ

in the progenitor cloud. For the sample of stars by Van Eck et al. (2003), obtained at lower res- olution, Eu was not measured. For these we as- sumed a typical [Eu/Fe]

ⁱⁿⁱ

= 0.5 as observed in unevolved halo stars. In Figs. 1 and 2 some ex- amples of stars fitted with updated AGB mod- els are shown. In Fig. 1, left panel, we report the star CS 22880-074 by Aoki et al. (2002a), with [Fe/H] = −1.93, fitted by an AGB model with a

¹³

C-pocket of ST/10 and a dilution fac- tor dil = 0.30 dex. The three lines give the un- certainties in the initial mass M = 1.20 ± 0.02 M . No r-process enrichment is necessary; see also Bisterzo et al. (2006) for the comparison of AGB model predictions with other stars by Aoki et al. (2002a). The right panel shows the star HE 0024-2523 by Lucatello et al. (2003), with [Fe/H] = −2.70, fitted by an initial mass

M = 1.3 M , dil = 0.0 and ST/12. In Fig. 2, left panel, the giant star CS 29497-34 ([Fe/H] =

−2.90) by Barbuy et al. (2005), is fitted with an initial mass M = 1.5 M



, dil = 1.0, [Eu/Fe]

ⁱⁿⁱ

= 1.5 and ST/4. In the same plot we compare the s+r prediction (solid line) with the model without r-enrichment (dashed line) where all the heavy element abundance predictions are from the s-process. The right panel shows the fit of the recent observed star HE 0338-3945 by Jonsell et al. (2006), [Fe/H] = −2.42, where a best fit is obtained for an AGB model of initial M = 1.3 M and ST/12. No dilution is neces- sary for this star that belongs to the main se- quence. In this case a strong pre-enrichment of r elements is adopted, corresponding to [Eu/Fe]

ⁱⁿⁱ

= 2.0. We note that Eu is a typical r-process element and that a typical [La/Eu]

s

≈ 1 dex is predicted. Other stars show similar r-enrichments (Tab. 1). The s-process in AGB stars produces very little Cr, Mn, and Cu. For the initial Cr and Mn we adopted the average value observed in unevolved stars in the same metallicity range (Franc¸ois et al. 2004). For Cu we assumed an initial solar-scaled abundance, although a subsolar value of [Cu/Fe] is in ac- cord with unevolved stars of low metallicity (Bisterzo et al. 2004). In Fig. 3 we report the [Pb/hs] predictions as compared with observa- tions. In the right panel stars are fitted by mod- els of 1.3 M and in the left panel by AGB models of 1.5 M . In Fig. 4, left panel, the s and r-rich star HE 0131-3953 by Barklem et al. (2005) is shown, with [Fe/H] = −2.71, fit- ted by an AGB model of 1.3 M



, ST/15, and [Eu/Fe]

ⁱⁿⁱ

= 1.5. For this star, as well as for all the other C-rich and s-rich stars in the sample of Barklem et al. (2005), lead was not mea- sured, and we give our predictions (see Tab.

2). In particular for HE 0131-3953, we predict a value [Pb/Fe] ≈ 3.1. The apparent discrep- ancies of C and N abundances may be partly reconciled by assuming a strong cool bottom process (CBP) occurring during the TP-AGB phase (Nollett et al. 2003; Wasserburg et al.

2006). Uncertainties in the spectroscopic anal-

ysis of lighter elements as C, N, O, Na, Mg and

Al in very metal poor stars have been exam-

ined in detail by Asplund (2005): 3D hydrody-

namical model atmospheres involve large dif-

-1 0 1 2 3 4 5

0 10 20 30 40 50 60 70 80 90

[El/Fe]

Atomic Number M ~ 1.2 M_o case: ST/10 [Eu/Fe]ⁱⁿⁱ= 0.0

CS22880-074

Aoki et al. 2002 [Fe/H] = -1.93 dil = 0.30

T_eff= 5850 K; log g = 3.8

C

N

Sr Y

Ba

La Ce Nd

EuDy Er

Pb

normalised to Lodders 2003 (solar photospheric)

n4 n3 n2

-1 0 1 2 3 4 5

0 10 20 30 40 50 60 70 80 90

[El/Fe]

Atomic Number M ~ 1.3 M_o case: ST/12 [Eu/Fe]ⁱⁿⁱ= 0.0

HE0024-2523

Lucatello et al. 2003 [Fe/H] ~ -2.70 dil = 0.0

C N

O

Na Mg

Al Si

T_eff= 6625 K; log g = 4.3

Ca Sc

Ti

CrMn Sr

Y Zr

BaLa

Eu

Pb

normalised to Lodders 2003 (solar photospheric)

n6 n5 n4

Fig. 1. Fits of two stars with updated AGB model predictions: CS 22880-074 (Left panel), Aoki et al. (2002a), and HE 0024-2523 (Right panel), Lucatello et al. (2003).

-1 0 1 2 3 4 5

0 10 20 30 40 50 60 70 80 90

[El/Fe]

Atomic Number M ~ 1.5 M_o case: ST/4

[Eu/Fe]ⁱⁿⁱ= 1.5 dil = 1.0 alpha0p5

CS29497-34

Barbuy et al. 2005 [Fe/H] = -2.90

C N

Na

Mg

Al CaTi

Cr Ni

Zn SrY

Ba La

Ce

Pr Nd

Eu Dy

Pb

T_eff= 4800 K; log g = 1.8 Thick line: [Eu/Fe]ⁱⁿⁱ= 1.5 Thin line: [Eu/Fe]ⁱⁿⁱ= 0.0

normalised to Lodders 2003

(solar photospheric) ⁿ²⁰n20 -1

0 1 2 3 4 5

0 10 20 30 40 50 60 70 80 90

[El/Fe]

Atomic Number M ~ 1.3 M_o case: ST/12 [Eu/Fe]ⁱⁿⁱ= 2.0 alpha0p5

HE0338-3945

Jonsell et al. 2006 [Fe/H] = -2.42 dil = 0.0

C N

O

Na Mg

Al

T_eff= 6160 K; log g = 4.1

V CaSc

Ti Cr

Mn Co

Ni Cu

SrYZr Ag

Ba La Ce

Pr Nd

Sm

Eu Gd

Tb

Dy Ho

Er Tm

Yb Lu Hf

Pb Th

U

normalised to Lodders 2003 (solar photospheric)

n6 n5 n4

Fig. 2. Left panel: the CS 29497-34, Barbuy et al. (2005), fitted by s+r process (thick line) and by s-process only (thin line). Right panel: HE 0338-3945, Jonsell et al. (2006), with a strong r-process enhancement.

-1 0 1 2 3 4

-3 -2.5 -2 -1.5 -1 -0.5 0 0.5

[Pb/hs]

[Fe/H]

Fit obtained by model of M= 1.3M

_o Lead stars Lucatello et al. 2005 Barklem et al. 2005, our Pb predictions

ST*2 ST*1.3 ST ST/1.5 ST/2 ST/3 ST/6 ST/12 ST/18 ST/24 ST/30 ST/45 ST/60 ST/75 ST/115 ST/150

-1 0 1 2 3 4

-3 -2.5 -2 -1.5 -1 -0.5 0 0.5

[Pb/hs]

[Fe/H]

Fit obtained by model of M= 1.5M

_o Lead stars Barklem et al. 2005, our Pb predictions

ST*2 ST*1.3 ST ST/1.5 ST/2 ST/3 ST/6 ST/12 ST/18 ST/24 ST/30 ST/45 ST/60 ST/75 ST/115 ST/150

Fig. 3. [Pb/hs] predictions versus metallicity for different ¹³ C-pocket compared with the spectro-

scopic data of s-rich and lead rich stars analysed in this paper. The two plots correspond to model

of M = 1.3 M  (Left panel) and M = 1.5 M  (Right panel). With empty squares we indicate lead

predictions for a selected sample of s-rich stars of Barklem et al. (2005).

Table 1. Stars sample. (

^∗

) [Eu/Fe] measured; (

^a

) Reference code at the end of citations

Lead Stars [Fe/H] [Pb/Fe] [hs/ls] [Pb/hs] M Pocket dil [Eu/Fe]

ⁱⁿⁱ

Ref

^a

CS 22183-015 -3.12 3.17 1.24 1.39 1.3 ST/15 0.0 1.5* [J2]

CS 22880-074 -1.93 1.90 1.03 0.71 1.2 ST/10 0.3 0.0* [A2a]

CS 22898-027 -2.26 2.84 1.30 0.67 1.3 ST/12 0.0 2.0* [A2a]

CS 22942-019 -2.64 ≤1.6 -0.13 ≤0.09 1.5 ST/75 0.0 0.5* [A2a]

CS 29497-030 -2.70 3.55 0.97 1.51 1.3 ST*2 0.4 0.5* [S4]

CS 29497-030 -2.57 3.65 1.04 1.42 1.3 ST*1.3 0.0 2.0* [I5]

CS 29526-110 -2.38 3.30 0.83 1.36 1.3 ST/6 0.0 1.5* [A2a]

CS 30301-015 -2.64 1.70 0.81 0.60 1.3 ST/24 1.0 0.0* [A2a]

CS 31062-012 -2.55 2.40 1.34 0.47 1.3 ST/30 0.0 1.5* [A2a]

CS 31062-050 -2.31 2.90 1.26 0.62 1.3 ST/12 0.0 1.8* [J4]

CS 31062-050 -2.42 2.81 1.55 0.59 1.3 ST/12 0.0 1.8* [A2a]

LP 625-44 -2.70 2.60 1.07 0.27 1.3 ST/30 0.0 1.8* [A2b]

HD 196944 -2.25 1.90 0.30 0.99 1.5 ST/3 1.8 0.0* [A2a]

HD 196944 -2.40 2.10 0.17 1.33 1.5 ST/3 1.3 0.5 [V3]

HD 26 -1.25 2.00 0.73 0.37 1.5 ST/2 0.8 0.5 [V3]

HD 187861 -2.30 3.30 0.6 1.33 1.5 ST/2 0.2 0.5 [V3]

HD 189711 -1.80 0.90 0.60 -0.70 1.3 ST/24 0.4 0.5 [V3]

HD 198269 -2.20 2.40 0.93 1.07 1.2 ST/9 0.4 0.5 [V3]

HD 201626 -2.10 2.60 0.70 1.00 1.5 ST/3 0.7 0.5 [V3]

HD 224959 -2.20 3.10 1.07 1.03 1.5 ST/2 0.3 0.5 [V3]

V-Ari -2.40 1.20 0.50 -0.40 1.2 ST/30 0.0 0.5 [V3]

HE 0024-2523 -2.70 3.30 0.56 1.67 1.3 ST/12 0.0 0.0* [L3]

HE 2148-1247 -2.30 3.12 1.10 0.87 1.3 ST/12 0.0 2.0* [C3]

CS 22948-27 -2.47 2.72 1.27 0.45 1.3 ST/24 0.2 1.5* [B5]

CS 29497-34 -2.90 2.95 0.98 0.87 1.5 ST/4 1.0 1.5* [B5]

HE 0338-3945 -2.42 3.10 1.30 0.79 1.3 ST/12 0.0 2.0* [J6]

ferences compared with 1D models, in some cases the discrepancies are of the order of 0.5 dex. The intrinsic indicator [hs/ls] is sensitive to the initial mass. In Fig. 4, right panel, there is an example of the predicted ratio [hs/ls]

versus [Fe/H], for various masses and a con- stant choice of the

¹³

C-pocket, ST/12. A nu- cleosynthesis indicator of the initial mass is given by Na and Mg: at very low metallic- ity an important primary production, which in- creases with the initial mass, is predicted for these two elements in AGB models. Indeed, a strong primary production of

²²

Ne results in the advanced pulses, by the conversion of pri- mary

¹²

C to

¹⁴

N in the H-burning ashes, fol- lowed by double α captures on

¹⁴

N in the ther- mal pulses, resulting in a primary production of

²³

Na via

²²

Ne(n,γ)

²³

Na, followed by a pri- mary production of Mg from

²³

Na(n,γ)

²⁴

Mg

and from

²²

Ne(α,n)

²⁵

Mg and

²²

Ne(α,γ)

²⁶

Mg.

Note that this primary production of light el-

ements extends up to P, including Al and Si

(see Fig. 5, left panel). In this figure we show

the production of light elements for different

masses at a given metallicity and Case ST for

the

¹³

C-pocket: there is a difference of up to

1.5 dex between AGB models of initial mass

1.3 and 1.5 M . The estimated absolute value

of the initial mass depends on the choice of the

mass loss rate. In the right panel our Zr and

Nb predictions are shown for extrinsic (solid

line) and intrinsic AGBs (dashed line). The s-

process Nb is the product of the radiogenic de-

cay of

⁹³

Zr (t

1/2

= 1.5 Myr), which is strongly

fed by the s-process. For intrinsic AGB stars a

[Zr/Nb] ≈ 1 is expected, whereas for an extrin-

sic AGB [Zr/Nb] ≈ 0. Consequently [Zr/Nb] is

a powerful nucleosynthesis indicator of an ex-

Table 2. Barklem et al. (2005), s-enhanced stars. (

^∗

) [Eu/Fe] measured; (

^b

) Pb prediction

Stars [Fe/H] [hs/ls] M Pocket dil [Eu/Fe] [Pb/Fe]

^b

[Pb/hs]

^b

HE 0202-2204 -1.98 0.72 1.5 ST*1.3 0.9 0.0* 3.0 1.8

HE 0231-4016 -2.08 0.65 1.2 ST*1.3 0.0 0.0 3.4 2.1

HE 1135+0139 -2.33 0.43 1.5 ST/6 1.8 0.0* 1.8 0.9

HE 2150-0825 -1.98 0.76 1.2 ST*1.3 0.0 0.0 3.4 1.9

HE 2240-0412 -2.20 1.13 1.3 ST/6 0.0 0.0 3.2 1.8

HE 0430-4404 -2.07 0.94 1.2 ST*1.3 0.0 0.0 3.4 1.9

HE 0131-3953 -2.71 1.51 1.3 ST/15 0.0 1.5* 3.1 1.1

HE 1105+0027 -2.42 1.46 1.3 ST/12 0.0 1.8* 3.0 0.8

HE 1430-1123 -2.71 1.40 1.3 ST/15 0.0 0.0 3.1 1.3

HE 2227-4044 -2.32 0.92 1.3 ST/6 0.0 0.0 3.2 1.9

-1 0 1 2 3 4 5

0 10 20 30 40 50 60 70 80 90

[El/Fe]

Atomic Number M ~ 1.3 M_o case: ST/15 [Eu/Fe]ⁱⁿⁱ= 1.5

HE0131-3953

Barklem et al. 2005 [Fe/H] = -2.71 dil = 0.0

T_eff= 5298 K; log g = 3.83

C

Mg

Al Ca

Sc Ti

Cr Mn

Ni Sr

Ba LaCe

Nd Eu

normalised to Lodders 2003

(solar photospheric) ⁿ⁶n5

n4

-1.5 -1 -0.5 0 0.5 1 1.5 2

-3 -2.5 -2 -1.5 -1 -0.5 0

[hs/ls]

[Fe/H]

[hs/ls] vs [Fe/H]

M = 1.3 M_o

M = 1.5 M_o

M = 2 M_o

M = 3 M_o

case ST/12

Fig. 4. Left panel: fit of HE 0131-3953, Barklem et al. (2005), where the lead is not measured.

Right panel: theoretical results for [hs/ls] versus metallicity for a

¹³

C-pocket of ST/12, and for different initial masses.

0 1 2 3 4 5 6

0 5 10 15 20 25

[El/Fe]

Atomic Number

[Fe/H] = -2.60 case ST

M = 1.3 M

_o

M = 1.5 M

_o

M = 2 M

_o

M = 3 M

_o

C

N O

NeNaMg

Al Si

P

Ca Sc

Ti

Cr Mn - - - -

-1 0 1 2 3 4 5

0 10 20 30 40 50 60 70 80 90

[El/Fe]

Atomic Number

M ~ 1.3 M

_o

case ST*1.3 [Eu/Fe]

ⁱⁿⁱ

= 2.0

[Fe/H] = -2.60

C N

O F Ne

Na Mg

Al Si

P S

Ca Sc

Ti Cr

Mn Co

Ni Zn

Sr Y

Zr Pd Nb

Mo Ag Ba

La Ce

Pr Nd

Sm Eu

Gd Tb

Dy Ho

Er Tm

Yb Lu

Hf Os Ir Pt

Pb Bi Nb

Zr Tl

Intrinsic AGB Extrinsic AGB

Fig. 5. Left panel: Light elements predictions for different solar masses, for the case ST at [Fe/H]

= −2.6. Right panel: Zr and Nb prediction for extrinsic (solid line) and intrinsic (dashed line)

AGB model of M = 1.3 M , [Fe/H] = −2.6, ST*1.3, [Eu/Fe]

ⁱⁿⁱ

= 2.0.

trinsic AGB in a binary system system, or of an intrinsic AGB star (Ivans et al. 2005).

3. Conclusions

We interpret C-rich, s-rich, lead-rich stars with low metallicity with AGB theoretical models of different s-process efficiencies and initial masses. The s+r enhanced stars are explained assuming that the parental cloud was triggered by a supernova pollution before the forma- tion of the binary system. In the sample there are some C and s-rich stars without observed lead and we give our theoretical prediction. A high [Nb/Fe] value is an indicator of an extrin- sic AGB in a binary system. The ratio [hs/ls]

may constrain the initial mass, while [Pb/hs]

and [Pb/ls] are indicators of the s-process ef- ficiency. Other indicators of the initial mass are the Na and Mg abundances, which are pro- duced in very low metallicity AGB stars by neutron capture on the very abundant primary

22

Ne. The introduction of an r-enrichment does not modify the ls, hs, and lead abundances.

Acknowledgements. MURST-FIRB project

”Astrophysical Origin of the Heavy Elements beyond Iron” and the Aspen Center for Physics.

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